Thanks. From that article: "To access an SS7 network, attackers can acquire an existing provider’s connection on the black (underground) market and obtain authorization to operate as a mobile carrier in countries with lax communications laws. In addition, any hacker who happens to work as a technical specialist at a telecommunications operator, would be able to connect their hacking equipment to the company’s SS7 network. In order to perform certain attacks, legitimate functions of the existing communication network equipment must be used. There is also an opportunity to penetrate a provider’s network through a cracked edge device (GGSN or a femtocell)."

So they don't have a way to break into WhatsApp without breaking into SS7 first. SS7 isn't publicly accessible, and it doesn't extend out to the air link for mobiles. The attacker has to find a backdoor into SS7. The problem is that, once on the SS7 network, you can send lots of messages which are believed by telco networks. You can divert phone calls and set up MITM attacks easily.

If this works against WhatsApp, their app lacks adequate MITM protection.